Getting correct Delta T readings

Had several problems and realized my plumber/installer had on idea on how to size a boiler or set it up proberly.
After many hours of reading the manual and boiler operation observation, and posting questions I am somewhat up to speed and have a handle on whats going on.
On e question if I may: I am trying to determine the actual D/T on my system. The system was plumbed with P/S loops. In reading the return temp on the screen am I not reading both the zone return water temp mixed with the boiler pump water also? If so then would this not would give a incorrect higher than actual zone return water temp?

If this is measuring IN the boiler, then yes, it's measuring what's already mixed from the primary and secondary loop(s) going back into the boiler. Probably the best way to actually see the individual loop return temps would be to install a thermometer at the end of each loop before it merges with the primary loop. If you have an IR thermometer, and you adjust it for the copper's reflectivity, that would work (or, get a reading on the outlet of the primary loop, compare to the boilers indication, and use a conversion factor, or just use that as the value when only looking for a delta-T). SOmetimes, it's easier to just put a section of black tape on the pipe, and read that instead (not all IR thermometers are adjustable and are calibrated for a 'typical' surface, not shiny metal).

I just thought it was strange that with all the talk of D/T on setting up a system properly that there is no easy way to do it on a P/S system.
I am going to try the black plumbers tape with my IR reader and see how that works. As long as the measurement is taken the same way, IR, Black tape, it may not be accurate as far as actual temp is concerned but the D/T should be.

To give you an idea of how much the actual material IR emissivity varies when using an IR thermometer, glance through this: http://www.scigiene.com/pdfs/428_InfraredThermometerEmissivitytablesrev.pdf. The better IR thermometers let you adjust the emissivity so you can get a more accurate reading...it can be very misleading if you take one calibrated for one thing, and then expect to get similar results with another.

How many feet of fin-tube baseboard (by zone, if broken up into zones)?

The delta-T on the baseboard loop will vary by water temp and flow rate, but unless you have a lot of it you won't be able to shed the boiler's min-mod output when running in condensing mode. It's unlikely that you're over-pumping the baseboard loop, but maybe. Even if i turns out you are pumping higher rates than necessary, the effect on net boiler efficiency or cycling would be small- you'd just be chewing through a lot more pumping kilowatt-hours than necessary to deliver the heat. But flow as low as 1 gpm or even at bit lower are usually fine for fin-tube baseboard.

Measuring the temperature of the pipe with an IR thermometer is quick, but none too accurate, if better than 2F accuracy matters to you.

I have found the hockey type tape rapped around the pipe seems to give consistant readings. The accuracy is off, in my case by 8-10deg, but the relitive temp reading is what I was after to measure the delta T.

The question I can't seem to get my head around is the change in D/T with change in secondary flow rates. It you reduce the flow rate on a zone, according to the slant fin chart, the BTU/foot will decrease. If you slow the flow rate and the btu dissapation decreases would not return water temp increase as you are extracting less btu's from the water that making the D/T lower?

The question I have that I can't seem to get my head around is the change in D/T with change in secondary flow rates. It you reduce the flow rate on a zone, according to the slant fin chart, the BTU/foot will decrease. If you slow the flow rate and the btu dissapation decreases would not return water temp increase as you are extracting less btu's from the water that making the D/T lower?

Tom

Click to expand...

Maybe this is a dumb question, but wouldn't it be a lot more efficient to not have primary and secondary loops at all? I understand the need if there is more than one zone, but if you pump the coldest water available back to the boiler isn't this the best?

I haven't used primary, secondary in years. I use hydronic separator s instead. Cheaper, and a whole lot faster. Less piping, less restriction and zero balancing and flow problems. Coupled with VFD circulators.

Hydronic separator, I did read somewhere about that. VFD circulators are variable volume is that right?

That might be a way to go. What I don't understand is the large Delta T's I read about and see in print. I do not see anything over 5-8 deg normally as read on the LCD on the Alpine boiler. Most people talk about 20 deg and more.

The Alpine manual has min flow rates on the primary loop based on Delta T. The values as published vary from 35* down to 20* with flow rates inverse to the D/T. How/where do you measure these temps. I don't see anything like that even with an infared temp gun on black friction tape rapped lines.

Can someone help clarify this for me? If I read the manual correctly and use my measured D/T's I am way off the charts as they only go down to 20*.

The reason your delta T is lower than expected is because the flow rate on the primary loop is higher than needed (under your test conditions). I don't think the low delta T is a problem as far as operation, it just means you are spending a little more on electricity than needed. If the volume of water is constant, the delta T is directly proportional to the BTUs put in or taken out.

Thanks for the reply.
I am reading the return water temp on the upstream side of a circulator in a given zone. The output temp right out of the boiler. The circulators are placed in the return side of the loop. I would think this is a good place to get the correct readings but they are not anywhere near the 20* D/T they are supposed to be. This would seem to be quitedetrimentall when trying to keep return water below 130* so as to operate in the condensing mode.

One problem I think I see is that the Kitchen loop is a total of slightly under 100'. If I am doing the math right using the total loop length X 1.5 to correct for elbows and such, X .04 gives me just under 6' of head. Using Taco's chart my 007 pump is flowing 13GPM. Can someone confirm this is correct?